1. Field of the Invention
The present invention relates to a semiconductor laser device in which light-emission end facets are coated with dielectric layers.
2. Description of the Related Art
Conventionally, in various types of known semiconductor laser devices, a semiconductor multilayer structure is formed on a substrate, and dielectric layers including a reflectance control layer are formed on two end facets of the semiconductor multilayer structure so as to realize resonator faces at the end facets. In these types of semiconductor laser devices, semiconductor materials are oxidized in vicinities of interfaces with the dielectric layers at both resonator faces, and often facet degradation occurs. Therefore, some techniques have been proposed for solving this problem.
Japanese Unexamined Patent Publication No. 58(1983)-125832 discloses that it is effective to remove oxidation layers produced on end facets of a semiconductor multilayer structure in a semiconductor laser device as mentioned above by use of various types of etching before surfaces of compound semiconductor materials are passivated.
U.S. Pat. No. 5,144,634 discloses a semiconductor laser device having oxygen free resonator faces and a technique for realizing the oxygen free resonator faces.
Japanese Unexamined Patent Publication No. 11(1999)-121877 discloses that it is possible to realize a state in which oxidation of resonator faces of a semiconductor laser device made of compound semiconductors is not detected by XPS (X-ray photoelectron spectrometry) analysis, when the resonator faces are processed with low-energy charged particles.
However, Japanese Unexamined Patent Publication No. 58(1983)-125832 does not disclose to what extent the natural oxidation layers should be removed, i.e., what oxygen content near-edge portions of the semiconductor multilayer structure (i.e., portions of the semiconductor multilayer structure in vicinities of resonator faces) should contain, in order to obtain a satisfactory result.
In addition, it is not proved that true oxygen-free state can be realized in the vicinities of the resonator faces by the technique disclosed in U.S. Pat. No. 5,144,634. Although the above technique may be able to realize a state in which a very small amount of oxygen remains, U.S. Pat. No. 5,144,634 does not disclose the actual amount of residual oxygen achieved by the technique.
Further, the present inventor made an investigation of the XPS analysis disclosed in Japanese Unexamined Patent Publication No. 11(1999)-121877, and found that SIMS (secondary ion mass spectrometry) can detect an unignorable amount of residual oxygen even when XPS cannot detect the residual oxygen. However, Japanese Unexamined Patent Publication No. 11(1999)-121877 does not report an actual amount of residual oxygen which can be achieved by the technique of Japanese Unexamined Patent Publication No. 11(1999)-121877 and cannot be detected by the XPS analysis.
The object of the present invention is to provide a reliable semiconductor laser device in which oxygen contents in portions of a semiconductor multilayer structure in vicinities of resonator faces are in a satisfactory range.
According to the present invention, there is provided a semiconductor laser device comprising: a multilayer structure including a plurality of semiconductor layers and being formed of a substrate; and at least one dielectric layer formed on each of two end facets of the multilayer structure, where the at least one dielectric layer on each of the two end facets includes a reflectance control layer. In addition, the oxygen content in at least one portion of the multilayer structure in at least one vicinity of at least one of the two end facets is 10 to 1,500 times higher than the oxygen content in the other portions of the multilayer structure.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (vii).
(i) The oxygen content in the at least one portion of the multilayer structure is 15 to 1,000 times higher than the oxygen content in the other portions of the multilayer structure.
(ii) The at least one dielectric layer comprises: a passivation layer formed directly on each of the two end facets of the multilayer structure; and the reflectance control layer formed on the passivation layer.
(iii) The passivation layer is made of at least one of Ge, Si, and C.
(iv) The passivation layer is made of an oxide containing at least one of Al, Ga, Si, Ge, Ta, and Ti.
(v) The passivation layer is made of a nitride containing at least one of Al, Ga, In, Si, Ge, C, Ta, and Ti.
(vi) The reflectance control layer is made of an oxide containing at least one of Al, Ga, Si, Ge, Ta, and Ti.
(vii) The reflectance control layer is made of a nitride containing at least one of Al, Ga, In, Si, Ge, C, Ta, and Ti.
Typically, the semiconductor laser device according to the present invention can be formed by using InGaN-based, ZnSSe-based, InGaAlP-based, AlGaAs-based, InGaAsP-based, InGaAs-based, and InGaSb-based compound materials, where the oscillation wavelengths of the InGaN-based compound semiconductor laser devices are in the range between 360 to 500 nm, the oscillation wavelengths of the ZnSSe-based compound semiconductor laser devices are in the range between 410 to 540 nm, the oscillation wavelengths of the InGaAlP-based compound semiconductor laser devices are in the range between 600 to 730 nm, the oscillation wavelengths of the AlGaAs-based compound semiconductor laser devices are in the range between 750 to 870 nm, the oscillation wavelengths of the InGaAsP-based compound semiconductor laser devices are in the ranges between 700 to 1,200 nm and 1,300 to 1,900 nm, the oscillation wavelengths of the InGaAs-based compound semiconductor laser devices are in the ranges between 950 to 1,200 nm and 1,300 to 1,900 nm, and the oscillation wavelengths of the InGaSb-based compound semiconductor laser devices are in the range between 1,800 to 3,000 nm.
The present inventor has found that the reliability of semiconductor laser devices is decreased when the oxygen content in near-edge portions of the semiconductor multilayer structure (i.e., portions of the semiconductor multilayer structure in vicinities of resonator faces) is very low as well as when the oxygen content in the near-edge portions of the semiconductor multilayer structure is high.
It is considered that when the oxygen content in the vicinities of the resonator faces is very low, the reliability is decreased for the following reasons (1) and (2):
(1) When oxidation layers on end facets of semiconductor laser devices are excessively etched by various types of etching as disclosed in JUPP No. 58(1983)-125832 in an attempt to more surely remove the oxidation layers, the end facets are damaged, and therefore the reliability is decreased.
(2) It is confirmed that a very small amount of oxygen remaining in the vicinities of the end facets of the semiconductor multilayer structure compensates, to some degree, for lattice defects caused by cleavage for producing the end facets. Therefore, when the oxygen content is too low, the lattice defects are not compensated for, and the performance of the semiconductor laser devices is decreased, i.e., the reliability is decreased.
In view of the above considerations, in the semiconductor laser device according to the present invention, the oxygen content in at least one portion of the semiconductor multilayer structure in the at least one vicinity of the at least one end facet, on which the at least one dielectric layer is formed, is set to 10 to 1,500 times (preferably 15 to 1,000 times) higher than the oxygen content in the other portions of the semiconductor multilayer structure. Therefore, the reliability (specifically, lifetime) of the semiconductor laser device according to the present invention is remarkably increased compared with the conventional semiconductor laser devices. The grounds of the above numerical limitations are explained later in conjunction with the embodiments of the present invention.